1. Technical Field
This invention relates to direct access storage devices (DASD) and more particularly to a system and method for reducing read access time to data which have been duplicated to two or more DASD units.
2. Description of the Related Art
A direct access storage device is an information storage device in which access time to data is essentially independent of the location of the data. The characteristic of independence in access time to data distinguishes direct access storage devices from serial storage devices such as tape drives. Disk drives of various types are the most common direct access storage devices.
A disk drive is a data storage device which utilizes at least one rotatable magnetic disk (or platter). The data are represented on the disk as a series a physical transformations of a recording media, such as magnetically polarized regions of a magnetic material. The physically transformed regions are arrayed along the surface of the disk along either a plurality of concentric data tracks or spiral data tracks.
A transducer reads data from a disk by producing an electrical signal in response to encountering the physically transformed regions. Data is written by using an electrical signal to control formation of the physically transformed regions along the various tracks on the disk while the disk moves underneath the transducer. In magnetic systems, the transducer forms a portion of a body called a slider. The slider flies just off the surface of a rotating disk. The slider is attached on its backside to a suspension system which in turn is connected to an actuator arm. The actuator arm is used to selectively position the head over one particular desired track during a read or write operation. As the disk is spun the magnetically polarized regions pass by the head one after another. The slider itself may be moved radially or on an arc inward or outward to change the track over which the slider is positioned.
While disk drive reliability has improved over the last few years, the devices are nonetheless electro-mechanical and liable to failure. These failures may be caused by a circuit defect which affects the readback function, in which case no data has been lost. Data may be recovered by repair of the circuitry to gain access to the data. If the failure occurs at an inconvenient time, the delay in recovery may be very expensive to user. If the failure occurs in the write circuitry or on the medium itself, then the data are permanently lost. If the failure is a so called head crash where a head strikes and destroys a portion of a medium surface, then data are permanently lost too.
The various causes listed above are characterized by the fact that generally only one DASD unit of a group in a system is affected at a time. Reliability in data access may be improved by use of more than one DASD unit to store redundant data. One redundancy scheme is called disk mirroring or disk duplexing. In mirroring, a record is stored on each of two or more DASD units. Each copy of a record is complete. Thus, in effect, two or more physical addresses (one for each drive) are provided for each logical address associated with the record.